全文获取类型
收费全文 | 352篇 |
免费 | 16篇 |
专业分类
化学 | 263篇 |
晶体学 | 1篇 |
力学 | 11篇 |
数学 | 31篇 |
物理学 | 45篇 |
无线电 | 17篇 |
出版年
2023年 | 2篇 |
2022年 | 10篇 |
2021年 | 12篇 |
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 12篇 |
2015年 | 13篇 |
2014年 | 8篇 |
2013年 | 18篇 |
2012年 | 21篇 |
2011年 | 16篇 |
2010年 | 9篇 |
2009年 | 8篇 |
2008年 | 22篇 |
2007年 | 16篇 |
2006年 | 17篇 |
2005年 | 17篇 |
2004年 | 5篇 |
2003年 | 14篇 |
2002年 | 11篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 11篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 8篇 |
1990年 | 2篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1941年 | 2篇 |
1934年 | 2篇 |
排序方式: 共有368条查询结果,搜索用时 22 毫秒
301.
Philippe Zeitoun Eduardo Oliva Marta Fajardo David Ros Stéphane Sebban Pedro Velarde 《High Energy Density Physics》2011,7(4):230-233
Seeding plasma-based soft x-ray laser (SXRL) demonstrated diffraction-limited, fully coherent in space and in time beam but with energy not exceeding 1 μJ per pulse. Quasi-steady-state (QSS) plasmas demonstrated to be able to store high amount of energy and then amplify incoherent SXRL up to several mJ. Using 1D time-dependant Bloch–Maxwell model including amplification of noise, we demonstrated that femtosecond HHG cannot be efficiently amplified in QSS plasmas. However, using Chirped Pulse Amplification concept on HHG seed allows to extract most of the stored energy, reaching up to 5 mJ in fully coherent pulses that can be compressed down to 130 fs. 相似文献
302.
303.
304.
Polymerizations of (R,S)-3,7-dimethyl-1-octene in the presence of heterogeneous or homogeneous isotactic specific catalysts give very similar products with a homopolymeric isotactic structure. The insertion of (R,S)-3,7-dimethyl-1-octene into the metal-CH3 bond (both heterogeneous and homogeneous catalysts) is highly stereospecific and stereoselective. 相似文献
305.
An almost alternating stereoregular copolymer can be obtained by copolymerizing ethylene and styrene with the catalyst ethylenebis(1-indenyl)zirconium dichloride activated by methylaluminoxane at −25°C. The regular microstructure pointed out by the NMR spectrum allows the copolymer to crystallize as shown by calorimetric and X-ray diffraction measurements. 相似文献
306.
Waldyr Muniz Oliva Jair Silvério Dos Santos Plácido Zoega Táboas 《NoDEA : Nonlinear Differential Equations and Applications》2000,7(3):259-283
It is proved the existence of a compact set , invariant under the flow of a Volterra system of retarded equations on , with lag r > 0; is homeomorphic to a solid tri-dimensional cylinder. The boundary of is the union of a closed bi-dimensional cylinder with two open disks (the two basis of the cylinder ). is the union of a continuous one-parameter family of r-periodic orbits of the retarded Volterra system and any r-periodic orbit of the retarded system is contained in . The flow, restricted to , of the system of retarded equations, is the flow of a C
1-vector-field. 相似文献
307.
V. Moliner J. Andrés M. Oliva V. S. Safont O. Tapia 《Theoretical chemistry accounts》1999,101(1-3):228-233
The present study elucidates structural features related to the molecular mechanism in the carboxylation step of the reaction
catalyzed by Rubisco. Starting from the initial X-ray Protein Data Bank structure of a Rubisco monomer, the reactive subsystem
in vacuo is subjected to quantum chemical semiempirical and ab initio studies, while the effects of the protein environments
are included by means of a hybrid quantum mechanical/molecular mechanical (QM/MM) approach. The QM/MM is used to characterize
the transition structure for carboxylation inside the protein. The calculations were made with the AM1/CHARMM/GRACE scheme.
Comparisons between the in vacuo and in situ transition structures show remarkable invariance with respect to geometric parameters,
index and transition vector amplitudes. The transition state couples the carbon dioxide attack to the C2 center of the substrate
in its dienol form with a simultaneous intramolecular hydrogen transfer from the C2 atom to the hydroxyl group linked to the
C3 center. This study suggests that carboxylation may be simultaneously coupled to the activation of the C3 center in the
enzyme.
Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 10 December 1998 相似文献
308.
J. Andrés M. Oliva V. S. Safont V. Moliner O. Tapia 《Theoretical chemistry accounts》1999,101(1-3):234-240
The carboxylation and oxygenation processes of a model substrate, 3,4-dihydroxy-2-pentanone, have been theoreticaly characterized
as a set of steps, mimicking the corresponding reactions of D-ribulose-1,5-bisphosphate catalyzed by rubisco. A theoretical characterization is carried out of transition-state structures
and possible molecular intermediates represented as saddle points of index 1 and minimum energy structures, respectively.
The quantum chemical characterization, at the HF/3-21G calculation level, of these stationary points is used to rationalize
and to discuss both catalyzed sequences. The reported set of these stationary points maps out most experimental aspects of
the reaction pathways for the real system.
Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 10 December 1998 相似文献
309.
We show that the control conditions in the feedback loop of a scanning tunneling microscope (STM) affect the values of the surface texture parameters, including fractal characterization, when they are calculated from STM images of the surface. The main surface texture parameters (such as the r.m.s. roughness, kurtosis, skewness, and average wavelength) show a strong dependence on the conditions used in the feedback loop for imaging. Fractal character changes also with feedback parameters. The quality of the STM images can be measured quantitatively by using some of the surface texture parameters. 相似文献
310.
Sulfated-zirconia aerogels were prepared by a one-step sol–gel synthesis combined with the supercritical drying technique using zirconium propoxide starting material with three different modifiers to control the hydrolysis reaction. The present study discloses a way to exert a significant degree of control on the morphology of zirconia sulfate catalysts. By an appropriate choice of the modifier one can deeply affect the porosity of these materials, in particular at low precursor concentration. 相似文献